Hydrophobically associated networks - Physical Gels from Biological and Synthetic Polymers - page 167

1000

100

10

1

0.1

10 12 14 16 18 20 22 24

End-cap length (carbon unit)

Effect of end-cap length on the Newtonian viscosity of an aqueous solution, c = 2% w/v of HEUR
(M w = 3.5 × 10 4 g mol − 1 ) at room temperature. Reproduced from Annable et al.( 1993 ) with
permission of the American Institute of Physics for The Society of Rheology.

Figure 6.7

6.3.5 Triblock copolymers with a hydrophobe core

Poly(ethylene oxide) n -poly(propylene oxide) m -poly(ethylene oxide) n (PEO n - PEO n )

triblocks, available commercially as Pluronic® (also called poloxamers), are non-ionic
macromolecular surface-active agents. They are an important class of surfactants
(Alexandridis and Hatton, 1995 ) with widespread industrial applications not only in
detergents, emulsi

PPO m -

cation, lubrication, cosmetics and inks, but also in pharmaceutical
applications for drug solubilization and controlled release.
The PEO n -

PEO n block copolymers are available in a range of molecular
masses and PPO/PEO ratios. The mass of the PPO group can vary between 950 and
4000 g mol − 1 , the PPO/PEO percentage varies between 10% and 80% and water
solubility increases with PEO content. The cloud point, the temperature at the
beginning of phase separation, for a polymer concentration of 1%, varies between
10°C and 100°C from low to high PEO content. Besides surface activity, these
copolymers have a

PPO m -

(viscosity or minimum concentration required
to obtain a gel at room temperature) which increases when the PPO block molecular
mass increases and the PPO/PEO ratio decreases. In practice, the rheology of con-
centrated solutions needs to be carefully examined (see the next section). Micelle
formation is observed with increasing temperature, and is associated with an endo-
thermic enthalpy change.

' thickening power '

6.4 Rheology of associating polymers

As already explained in Chapter 4 , associating polymers have non-permanent (labile)
junctions, giving rise to some interesting solution rheology. At low shear rates, the
solutions exhibit liquid-like behaviour with a Newtonian plateau, but the main interest
in applications of associating HM water-soluble polymers is the very important increase
of the zero-shear viscosity compared to that of the unmodi ed polymer.